There are circumstances in which it is important to be able to mix bubbles of air or other gas into a body of liquid in order to treat it. One well-known instance is waste water treatment. Industrial effluents and sewage need aeration. Less well known but of great importance is the treatment or aid to natural regeneration of large static water supplies, such as rivers, lakes and reservoirs. Particularly where there is little or no natural flow or circulation, such large bodies of water are liable to certain difficulties which can render them unfit for use unless special measures are taken. In particular, bodies of water tend to stratify stably into layers of different temperatures, which do not mix with one another and which contain different levels of dissolved oxygen. Water from lower layers generally contains very little dissolved oxygen. It may contain high levels of dissolved metals or other pollutants. It is usually not fit for use. This is a problem in a reservoir if the level falls. Another issue is the growth of algae, particularly blue-green algae, which is an undesirable presence in reservoirs and flourishes at certain levels in still water, particularly the sunlit surface layers where it consumes oxygen (“BOD”). Chemical reactions occurring at lower or bed level may also consume oxygen (“COD”). The sea is likewise prone to stratification, despite tidal flows, e.g. there can be problems for fish farms and shellfish farms when concentrations of undesirable organisms such as algae arise near or are carried into the farm zone in a stable stratum.
Over the years there have been various proposals for dealing with these problems. One approach is to generate a line source or point source of bubbles by pumping compressed air through a series of holes in a pipe lying on the bed, or through a porous block. The rising bubbles entrain water and generate a buoyant bubble plume—a mixture of water and bubbles—which causes vertical exchange and mixing, reducing the bad effects of thermal stratification. A refinement of this is to provide an upright tube near the bottom of the body of water and pump compressed air into the bottom of this tube, in the manner of an air lift pump used in dredging. The resulting imbalance of hydrostatic pressure forces the low-density air/water mixture continually up the tube, creating a substantial upward flow into which (a secondary benefit) some extra oxygen may dissolve. The air/water mixture leaving the top of the tube entrains further near-bed water as it rises towards the surface, increasing the vertical exchange effect. The best entrainment is achieved when the bubbles are small and evenly distributed in the tube and the flow is strongly turbulent and rotational (swirling). U.S. Pat. No. 3,452,966 (Smolski) describes a device known commercially as the “Helixor” in which the cylindrical tube interior is spanned by an integrally-extruded helically-twisted strip, to cause a rotational flow when compressed air is directed into the two openings at its lower end from holes in an air line. This device has been widely used over the years. However it is difficult to make, and the air bubbles tend to be large and to follow the shortest path up the helix without mixing with the water. Also the flow resistance is high. See also EP-A-826640. Other prior proposals use a draft tube with an empty interior, either without rotation or creating rotation flow by the circumferential angling of compressed air input jet openings near the base of the column. See WO 79/00895 and U.S. Pat. No. 3,855,367. However the rotational impetus in the latter is small.
In the waste treatment field, many gasification apparatus are described using rotating impellers or paddles to drive mixing between liquid and gas, but the need for a mechanical drive in situ makes these expensive and limited in their field of use.